Your wallet is packed with cards, but add another: a prescription drug discount card.

A recent ConsumerWorld.org survey found that people can save significant money using little-known drug discount cards, especially free ones. The trick is figuring out which card saves the most.

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ConsumerWorld checked the prices of four popular drugs, two brand-name and two generics, using five different discount cards at CVS, Costco and an independent pharmacy. Savings ranged from 0 to 71 percent off the cash price an uninsured person would pay. The average: 16 percent.

Prescription drug discount cards are a membership benefit of organizations like AAA and AARP, and are also available free online. Some chains sell cards for use at their pharmacy.

"Consumers buying a prescription drug not covered by insurance would be crazy not to get one (or more) of these cards," says ConsumerWorld's Edgar Dworsky. "Start with the NeedyMeds.org card, and print one or two from the Internet. Google 'prescription drug discount cards.' "

The survey's biggest surprise? Costco's cash price was often cheaper than any card. The cards are best for generics, says Gabriel Levitt of PharmacyChecker.com in White Plains. "The discounts are often small, 5 to 10 percent, for brand-name drugs."

Because savings vary (in some cases, a card may even be higher than the noncard price), ask your pharmacist to check which card offers the best deal. Before getting a card, make sure the drugs you're likely to buy will be discounted, and find out if your pharmacy accepts it, adds Carrie McLean of eHealthInsurance.com.